There is an increasing demand of miniaturizing high-frequency filters for use in portable radio communication apparatuses such as mobile telephones. Such a high-frequency filter should have a good frequency selectivity and at the same time must be able to be manufactured at low cost. There has been proposed, as a high-frequency filter which meets the above demands, a ceramic filter of the multilayer structure in which stripline electrodes are arranged as resonators (refer, for example, to WO96/19843). This type of dielectric filter is advantageous in that its size can be reduced since effective wavelengths of the signals used therein become shorter by virtue of the high dielectric constant of the ceramic dielectric materials used, whereby the lengths of the resonators can be shorter.
A dielectric filter of the above type in which dielectric materials of high dielectric constants are used, however, has a disadvantage that its frequency characteristics are largely affected by a small change in size of the electrodes provided therein. For this reason, dielectric constants of dielectric materials used in this type of dielectric filters are limited by a certain upper value which typically is about 100. As a dielectric filter which can further be reduced in size with a dielectric material having such a limited dielectric constant, a dielectric filter of the so-called SIR (Stepped Impedance Resonator) type having resonator electrodes of specially designed shapes has been proposed, for example, in Japanese Patent Application Laid-Open No. 7-312503. Each resonator of the SIR type comprises a narrow first resonator portion (of high impedance) which is grounded at its proximal end and a wider second resonator portion (of low impedance) which adjoins a distal end of the first resonator portion, the second resonator portion being open at its distal end. The resonators of such SIR type can be shorter at the same frequency, so that the filter can further be reduced in size. However, the dielectric filter of the above-described SIR type is disadvantageous in that concentrations of currents at the narrow first resonator portions of the resonators result in a substantial loss, which causes the insertion loss of this filter to increase.